The present invention relates to a method and apparatus for adding a fluid additive to a fluid. The invention relates particularly, but not exclusively, to a method and apparatus for securely adding an additive to fluid dispensed from a delivery means which is required to deliver fluid with and without an additive. The invention also relates particularly, but not exclusively, to a delivery means which is a delivery vehicle and to a method for securely adding marker chemical to middle distillate oils.
Many countries impose different rates of taxation on particular grades of middle distillate oils. For example, diesel grade oil may be taxed at a relatively high rate when used for powering on-road vehicles, but be untaxed or taxed at a lower rate when used for heating purposes, or for powering off-road vehicles. Where such variations exist, it is necessary for the taxation authority to ensure that the untaxed or lower taxed oil cannot be used in circumstances where the higher tax rate should apply. Various methods have been used to meet this need. One method requires the users of higher taxed oil to keep records of distances travelled by means of a special meter and account for tax on this basis from time to time. Another more common method involves collecting the tax on the higher taxed oil at source and chemically marking the untaxed or lower taxed oil in order that any prohibited use can be readily detected.
Chemical marking usually takes place at the refinery or bulk storage depot. It typically comprises two main components, a coloured dye marking chemical which provides readily recognised marking on visual inspection and an invisible second marking chemical which is such more difficult to remove than the dye chemical.
The use of chemical marker, or marker, has several advantages over the metering method. Firstly, it is more easily controlled. Secondly, it eliminates the need for special meters. Thirdly, it eliminates the need to record and account for distances travelled. Fourthly, it taxes fuel consumption rather than distance travelled and therefore encourages fuel efficiency.
However, it has the relative disadvantage that the same delivery vehicle will not normally be able to deliver both marked and unmarked oil where the oil is stored in a common tank on the vehicle. This arises because the systems available for the addition of marker are unlikely to be sufficiently secure to be accepted by the taxation authorities. For example, with systems which are currently available, a dishonest operator could deliver unmarked oil but record it as marked and thereby avoid payment of the higher tax by either disabling the marker system or by replacing the marker with a different fluid. Throughout this specification, the term secure refers to an acceptable level of prevention of tampering or unauthorised interference.
The requirement to use different delivery vehicles for marked and unmarked oil may increase costs in several ways. Firstly, it may necessitate the need for larger numbers of delivery vehicles. Secondly, it may necessitate additional distances travelled in situations where one destination or route could be delivered by one vehicle if it could deliver both types.
An object of the invention is to overcome these disadvantages by providing a secure system which can add marker at the point of delivery and thus allow one vehicle to deliver both marked and unmarked types.
It is noted that the disadvantages associated with current systems would not be adequately overcome by using a delivery vehicle with two or more tank compartments, separately containing marked oil and unmarked oil, for the following reasons. Firstly it may be troublesome to attempt to match the relative quantities for marked and unmarked oil on the vehicle with the relative quantities required for its delivery schedule. The relative quantities may not even be known in advance. Also, quantities can only be carried in discrete tank sized amounts. Secondly, changing tank use from marked to unmarked use may necessitate frequent cleaning of the tanks, which would be time consuming and costly, and would carry the risk of residual marker contaminating the unmarked oil. Thirdly, where the risk of cross contamination prevented sharing of delivery equipment for marked and unmarked oils, carrying the additional type of oil would further increase costs because it would necessitate equipping the vehicle with an additional pump and flowmeter.
The possibility of cross contamination from shared delivery equipment is also important in the general case of a delivery vehicle with a larger number of separate compartments holding the same basic fluid but with different additives. Such vehicles frequently measure quantities delivered by means of simple dipsticks because a common flowmeter cannot be used due to the possibility of cross contamination and it would be too expensive to provide flowmeters on every compartment. Measurement by dipstick is costly in labour and can be difficult in poor weather conditions. A further object of the invention is to overcome this disadvantage by providing a secure system which can add additive at the point of delivery and thus allow a range of fluid and additive mixtures to be delivered by common equipment, including a flowmeter, without risk of cross contamination.
Additive injection is used to inject fluid additives into base fluids in measured proportions. Reciprocating injection pumps, or injectors, are commonly used as dosing pumps in apparatus used for additive injection.
The injector typically comprises a piston and cylinder arrangement which is provided with an inlet check valve and outlet check valve and a means for reciprocating the piston in the cylinder. When the piston is drawn back in the cylinder, the negative pressure differential created in the cylinder causes the inlet check valve to open and the outlet valve to close or remain closed and additive is drawn into the cylinder through the inlet pipe. When the piston returns in the cylinder, the positive pressure differential created causes the inlet valve to close or remain closed and the outlet valve to open and additive is expelled from the cylinder into the outlet pipe. This process is repeated at each cycle of the injector.
The reciprocating means can be provided in several different ways, a very common means being by the use of a pneumatic piston and cylinder actuator which has its piston coaxial with and linked to the piston of the injector. The pneumatic actuator piston may be reciprocated by conventional pneumatic control means which in turn reciprocates the injector piston. Other examples of reciprocating means include spring returned pneumatic actuators and mechanical eccentric cams driven by rotating means.
Where a pneumatic actuator is used, the operation of the injector actuator and pump will usually be triggered by a pulse or signal from a device associated with a flow meter measuring the flow of base fluid and which will cause the injector to carry out one reciprocating cycle comprising a suction and delivery stroke. If the pulse or signal from the device is arranged such that it occurs each time a set proportion of base fluid passes the flow meter, then the flow of additive pumped by the injector will be proportional to the flow of base fluid. The reciprocating cycle is conventionally seen to have a characteristic length of time for each injector which will determine the maximum rate at which the injector can be run.
Usually some means is provided whereby the volume displaced at each stroke of the injector piston can be varied by varying the length of the piston stroke. Where a pneumatic actuator is used, this variation is frequently provided by a threaded adjustment member which acts as a stop which limits the length of the piston stroke in one direction. This variation allows the pump to be calibrated subsequent to manufacture.
The accuracy of injectors of the type described above, across the range of working pressures, can vary up to about xc2x15%. Where greater accuracy is required, other means are frequently used, such as proportional metering valve arrangements with direct flow meter control which can readily give absolute accuracies across the range of working pressures within xc2x10.5%. The metering valve arrangement has the disadvantage that it is usually more complicated and expensive than an injector.
Injectors can also have the disadvantage that they do not mix additive evenly into the base fluid but inject discrete amounts of additive into a continuous stream of base fluid. This intermittent mixing method, sometimes referred to as slugging, gives rise to two potential problems. Firstly, the injected stream comprising additive and base fluid does not initially form an even mixture. Secondly, where small batch quantities are taken from the uneven mixture, the overall resulting proportions may be significantly incorrect and cannot be rectified by subsequent mixing of the batch quantities.
It is also an object of the present invention to provide apparatus which reduces the slugging problems which can arise from the uneven mixture of additive and base fluid caused by the injection method. It is a further object of the present invention to provide apparatus which can provide much improved accuracy in relation to the proportion of additive injected into the base fluid.
The invention is defined in the appended claims 1 to 13 which are incorporated into this description by reference.